The goal of this application is to identify the molecular mechanisms of perinatal HIV-1 transmission with the aim of developing relevant strategies for prevention. The investigator posits that there are specific molecular and biological properties of HIV-1 that include the functional domains in the gp120 region of Env and the p17 region of Gag that are critical determinants of perinatal HIV-1 transmission. Hypothesis-driven studies are proposed to; determine the molecular properties of viruses transmitted from an infected mother to her infant, identify and characterize the viral determinants associated with perinatal HIV-1 transmission, and determine the biological properties of viruses that are or are not transmitted from a mother to her infant.